Answer
True
Work Step by Step
Given: $x'(t)=A(t)x(t)+b(t)$
To form a matrix multiplication $A(t)x(t)$, both $A(t)$ and $x(t)$ must have $n$n columns. Furthermore, $x'(t)$ is also a column $n$ vector function, hence $A(t)$ must have $n$ rows as well.
The given mstatement is true.